1. Field of the Invention
The present invention relates to a shock absorber damping force control system for a vehicle.
2. Description of the Prior Art
Various damping force control systems for automotive shock absorbers are known as disclosed in Japanese First (unexamined) Patent Publication No. 3-104726. In that publication, the system judges a direction of a required damping force to be produced based on comparison between signs of a monitored sprung mass velocity and a monitored relative velocity between the sprung mass and an unsprung mass, and further judges a stroke direction of the shock absorber based on the sign of the relative velocity. The system varies the damping force based on these decisions so as to provide both driving stability and riding comfort. This system utilizes the theory known in this field of endeavor as "skyhook". According to this system, however, since damping or suppressing low-frequency vibration of the sprung mass is preferentially dealt with, high-frequency vibration is increased when the vehicle travels on an unlevel or untreated road surface, resulting in (1) the of vehicular passengers having a feeling like they are running on a hard rugged road surface; (2) poor treading of the tire on the ground. Furthermore, when passing over an obstacle on the road surface, the vehicular passenger's feeling of running on a hard rugged road surface is increased.
In order to improve the problems, with the system of the type disclosed in Japanese First (unexamined) Patent Publication No. 3-227711, as noted above, a signal from sprung mass acceleration detecting means is Fourier-converted to derive power spectrum data for deriving the respective sums of the power spectra across the resonance points of the sprung and unsprung masses. Based on the derived sums, a hardness value of the damping force is corrected so as to improve the suppression of the sprung mass vibration, the riding comfort and the treading of the tire on the ground.
Japanese First (unexamined) Patent Publication No. 60-183216 discloses another type of the shock absorber damping force control system. In that system, in order to reduce a shock generated when the vehicle passes over an obstacle on the road surface, a damping force for rear wheels is maintained low from when front wheels sense the shock, and is returned to the original value after the rear wheels pass over the obstacle.
In the aforementioned publications, however, sufficient consideration has not been given as to how a vehicular passenger feels when vibration is caused by both the sprung mass resonance point and the unsprung mass resonance point. The sprung mass resonance point causes the vehicular passenger to have a wavering feeling due to a low-frequency vibration of the sprung mass (hereinafter also referred to as "on-wave feeling", "on-wave behavior", "on-wave vibration" or "on-wave component") The unsprung mass resonance point causes the vehicular passenger to have a feeling of running on a hard rugged road surface due to a high-frequency vibration of the sprung mass (hereinafter also referred to as "on-ruggedness feeling", "on-ruggedness behavior", "on-ruggedness vibration" or "on-ruggedness component"). Nor do the aforementioned publications discuss how to control the damping force under the condition that both resonance points are input so as to provide a better feeling to the vehicular passenger, and how a vehicle's speed affects the feeling of the vehicular passenger, resulting in poor improvement of the riding comfort.
On the other hand, in the second aforementioned publication, although the system works well when the obstacles in the road are independently arranged at constant intervals, such as on the high level roads, it does not work well when obstacles exist on a largely waving road. Specifically, under this condition, keeping the damping force at a low level causes vehicle body vibrational behavior to increase, which should be avoided. Further, when the obstacles exist on an unlevel road surface in a continuous manner at intervals shorter than a vehicular wheel base, the damping force is kept low which deteriorates the treading of the tire on the ground and the driving stability of the vehicle.